Systems and methods for damage detection

ABSTRACT

A system for detecting damage to a glass surface particularly vehicle glazing panels such as vehicle windscreens. The system uses a sensor unit disposed proximate the surface and a processor in communication with the sensor unit. The processor is configured to analyse data received from the sensor unit in order to determine the integrity of the surface and a communication unit is configured to output a signal in response to the processor determining that the surface has been damaged. For vehicle glass the system is preferably integrated into the vehicle management and control systems such that the system is active when the vehicle is active or moving. The management and or control system may monitor for instances or situations when changes, such as above threshold changes, occur in order to produce an output warning signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/483,356 filed on Aug. 2, 2019, which is the National Stage ofInternational Patent Application No. PCT/GB2018/050271 filed on Jan. 31,2018, which claims priority from British Patent Application No. GB1701924.1 filed on Feb. 6, 2017, each of which is hereby incorporated byreference herein in its entirety.

BACKGROUND 1. Field

The present invention relates to systems and methods for detectingdamage to a glass surface. In particular, the invention provides systemsand methods for detecting surface flaws or chips on a window pane, suchas a vehicle windscreen.

2. State of the Art

Vehicle windscreens are typically made of a laminate constructioncomprising two plies (or layers) of glass separated by an interlayer,usually made of a plastics material. A vehicle windscreen can easilybecome damaged by a flaw or chip on the outer ply. The most common causeof such a chip is a stone or other small projectile impacting thewindscreen whilst the vehicle is being driven. Similar damage can alsooccur to other glass surfaces, such as doors, windows and light covers.

These chips are generally of the order of up to 1 cm in size wheninitially created, depending on the impact speed and the temperature ofthe windscreen. A chip of this size in the outer surface of thewindscreen can often be repaired without requiring replacement of thewindscreen, for example by injecting resin into the damaged area. Thisdepends on many factors, such as the size, depth and location of thedamaged area, and local standards regarding windscreen repair.

A surface chip will increase in size after creation if the vehiclecontinues to be driven, due to direct stress and static fatigue. Driversoften do not notice, or become concerned about a chip until it hassubstantially increased in size. If left unrepaired a small chip canoften turn into a substantial crack in the windscreen which is notrepairable, requiring the entire windscreen to be replaced. This processis much more expensive and time consuming than simply repairing theinitial chip. It is therefore desirable to identify any chips in awindscreen and ensure that they are repaired as soon as possible aftercreation.

As small surface chips or flaws in a glass surface can be difficult toidentify by eye, there is a need for a system which automaticallydetects surface chips or flaws in a glass surface. This may be moreconvenient and efficient that relying on manual detection methods.

SUMMARY

In a first aspect of the present invention there is provided a systemfor detecting damage to a glass surface, the system comprising:

-   -   a sensor unit disposed proximate the surface;    -   a processor in communication with the sensor unit, wherein the        processor is configured to analyse data received from the sensor        unit to determine the integrity of the surface; and    -   a communication unit configured to output a signal in response        to the processor determining that the surface has been damaged.

This system provides the advantage that any damage to the surface isautomatically detected and communicated to a user, which is faster andmore accurate than relying on a user to see the damage and to act onthis.

Optionally, the glass surface may be a vehicle windscreen, or a side orrear vehicle window. In some embodiments, the glass surface may comprisetoughened, or semi-toughened glass.

For vehicle glass the system is preferably integrated into the vehiclemanagement and control systems such that the system is active when thevehicle is active or moving. The management and or control system maymonitor for instances or situations when changes, such as abovethreshold changes, occur in order to produce an output warning signal.

In some embodiments, the sensor unit may comprise a microphone. Theprocessor may comprise a sound amplification and signal processingsystem. The sound of a stone or other small projectile impacting thesurface will result in a signal output from the microphone which has aparticular time response and frequency spectrum. The processor may beprogrammed to identify a plurality of predetermined signals indicativeof damage events. The processor may activate the communication unit whenone of these predetermined signals is recorded (i.e. output from themicrophone).

Additionally of alternatively, the processor may then activate anothercomponent of the sensor unit to perform a secondary or back-up test.

Optionally, the sensor unit may comprise a camera. The camera may bemounted such that it has a complete view of the surface. Optionally, thesensor unit may comprise a controller configured to move, tilt or pivotthe camera. The camera may therefore be configured to scan the surface.

The processor may comprise image processing software which analyses theimage of the surface received from the camera to identify any possibledamage areas, such as a chip.

The camera may be constantly in operation (i.e. continually imaging thesurface). Optionally, the camera may be activated periodically. This mayadvantageously be more energy efficient and conserve the power supply ofthe camera. For example, when the surface is a vehicle windscreen orother vehicle window, the camera may be automatically activated inresponse to the ignition starting the vehicle's engine.

Optionally, the processor may be operable to communicate with a vehiclewindscreen cleaning system. For example, the processor may be able toinstruction the windscreen wipers to clean the windscreen prior toactivating the camera. This may reduce the risk of any dirt or debris onthe windscreen being mistaken for a damaged area by the image processingsoftware.

In some embodiments, the sensor unit may comprise both a camera and amicrophone. The camera may be activated by the processor receiving apredetermined signal from the microphone indicative of a damage event.Thus, the camera may be used to verify or confirm that the windscreenhas been damaged, and/or to identify the location of the damaged area.This is advantageous as it improves the accuracy of the detectionsystem, as although the microphone may indicate that an object hasimpacted the surface, this may not have resulted in any damage. Thecamera may then prevent a user having to conduct a visual inspection ofthe surface, which can be inconvenient and time consuming.

Optionally, the communication unit may be configured to output one ormore images received from the camera, optionally via a data link.

In some embodiments, the sensor unit may comprise an optical transmitterand an optical detector. The optical transmitter and the opticaldetector may be disposed in a fixed position, or they may be moveablerelative to the surface. For example, the optical transmitter and theoptical detector may scan across the surface. The movement of theoptical transmitter and/or the optical detector may be controlled by acontroller.

The optical transmitter may be configured to at least partiallyilluminate the surface and the optical detector may be arranged to atleast partially receive the light output from the optical transmitter.Thus, the optical detector may record the amount of light reflected by,or transmitted through, the surface.

Optionally, the optical transmitter may be arranged to at leastpartially illuminate the external surface of a windscreen, and/or theinterlayer of a windscreen.

A flaw or chip in the surface will disrupt or attenuate the light outputby the optical transmitters. This disruption will be identified by theprocessor, wherein the processor may then activate the communicationunit to output a signal indicating that a damage event has taken place.Additionally of alternatively, the processor may then activate anothercomponent of the sensor unit to perform a secondary or back-up test.

Optionally, the sensor unit may comprise an array of opticaltransmitters. The array of optical transmitters may be configured tooutput a dispersed light pattern. The angle of incidence of light outputby the array of optical transmitters may be selected such that the lightundergoes total internal reflection (TIR) through the surface. Inembodiments where the surface is a windscreen, the light may undergo TIRalong the outer ply, or along the interlayer between the outer and innerplies. For example, the array of optical transmitters may be positionedinternally of the windscreen. A flaw in one of the plies will result inincreased light leakage through the flaw.

The optical transmitter(s) and optical detector(s) (i.e. the opticalsystem) may operate continuously. Optionally, the optical system mayoperate periodically. For example, the optical system may operate incombination with the microphone and/or camera. The optical system maytherefore verify or confirm the data recorded by the other components ofthe sensor system.

In some embodiments, the detection system may comprise an electricallyconductive film or coating disposed on the surface. The sensor unit maycomprise an electric circuit which is operable to measure the electricalresistance of the film. For example, the electric circuit may comprise avoltmeter. The electrical resistance of the coating will change if thesurface becomes damaged and this change can be detected by theprocessor.

Optionally, the film may be disposed on the outer (i.e. external) faceof the surface. This makes the film more sensitive to even small flawsor chips in the outer face of the surface, but the lifetime of the filmwill be decreased as it will degrade faster due to being exposed toexternal factors such as the weather and the action of windscreen wipersetc. The processor may be configured to instruct the communication unitto output a signal after a predetermined amount of time to warn a userthat the film should be replaced.

The conductive film may be used in combination with any other disclosedfeatures of the sensor unit.

In some embodiments, the sensor unit may comprise a transducerconfigured to output acoustic waves which vibrate the surface and areceiver arranged to measure the vibration of the surface. Thetransducer and/or the receiver may comprise piezoelectric material.Optionally, the transducer and the receiver may be the same device whichoutputs pulses of acoustic waves. In some embodiments an array oftransducers and receivers may be provided.

When undamaged the surface will vibrate with a specific acoustic signaland temporal profile that can be predetermined (e.g. based on theproperties of the surface). When damaged the acoustic signal (orvibration) of the surface will be altered, for example the frequencyspectrum may be permanently changed compared to the undamaged signal.These changes will be detected by the processor. The processor may thenactivate another component of the sensor unit to verify that the surfacehas been damaged. Additionally or alternatively, the processor mayinstruct the communication unit to output a signal (e.g. an alert).

The processor may require multiple acoustic (i.e. vibration) signalsindicative of a damage event to be recorded over a certain time framebefore taking any further action. This may prevent signals or alertsbeing output due to rain or hail or other objects causing the surface totemporarily vibrate without causing damage.

The acoustic vibration system could be used in combination with anyother disclosed features of the sensor unit. This may improve theaccuracy and/or reliability of the detection system.

The system may comprise a data storage device. The data storage devicemay be configured to store data output from one or more of the sensorunit, the processor and/or the communication unit. The data storagedevice may be removable, for example a memory card or disc.

The communication unit may be configured to output a visual and/or audioalert that a damage event has occurred. Optionally, the communicationunit may be operable to output a signal (e.g. an alert) to a remotelocation. The remote location may be a repair centre which can thenautomatically schedule the surface to be repaired. This may beadvantageous as it eliminates any inconvenience for the user (e.g.driver) and prevents any delay in getting the damage repaired, which cansave the user money as a minor repair often becomes a much larger moreexpensive job if neglected.

The communication unit may comprise a transceiver for outputting thesignal, for example an RF transceiver. Optionally, the communicationunit may be configured to output a signal via a wireless data link.

Optionally, the communication unit may comprise a GPS transmitter and aGPS receiver. In some embodiments, the communication unit may output analert to a designated repair centre, regardless of the location of thevehicle. The contact details for the designated repair centre may beprogrammable by the user. In other embodiments, the communication unitmay contact the nearest repair centre to the vehicle, for example if thewindscreen has cracked or if the repairs required are significant.Optionally, the communication unit may contact the nearest branch of adesignated chain of repair centres.

In some embodiments, the communication system may be in communicationwith the vehicle alarm system (e.g. the intruder alarm system). Forexample, if the system detects a crack or a damaged area of thewindscreen which exceeds a threshold size or severity then thecommunication system may activate the vehicle alarm system. Conversely,if the vehicle alarm system is activated due to a window breaking in thevehicle then the communication system may output an alert.

In a second aspect of the present invention, there is provided a methodfor detecting damage to a glass surface, the method comprising:

-   -   measuring one or more acoustic, electrical, or optical        properties of, or proximate to, the surface using a sensor unit;    -   analysing data received from the sensor unit using a processor        to determine the integrity of the surface;    -   outputting a signal from a communication unit in response to the        processor determining that the surface has been damaged.

In particular embodiments, the glass surface may be a vehiclewindscreen, or a side or rear vehicle window. In some embodiments, theglass surface may comprise toughened, or semi-toughened glass.

The sensor unit may comprise a plurality of sensors. In someembodiments, the method may comprise the step of instructing the sensorunit to verify whether the surface has been damaged by conducting afurther measurement using a different sensor.

In some embodiments, the step of measuring one or more properties usinga sensor unit may comprise recording the sound of a potential damageevent using a microphone, and converting this sound signal into anelectrical signal. The method may include identifying whether theelectrical signal matches one of a plurality of predetermined signalsindicative of damage events.

Optionally, the step of measuring one or more properties of the surfaceusing a sensor may comprise imaging the surface using a camera. Themethod may also include moving the camera to scan across the surface.The step of analysing data received from the sensor unit may compriseusing image processing software to analyse one or more images of thesurface to identify any possible damage areas, such as a chip.

Optionally, the method may include cleaning the surface prior toactivating the camera.

In some embodiments, the method may include activating the camera inresponse to the microphone (or other sensor) recording a signalindicative of a potential damage event. Thus, the camera may be used inthe verification step.

Optionally, the method may include displaying the signal or alert outputby the communication unit.

In some embodiments, the step of measuring one or more properties of thesurface using a sensor unit may comprise at least partially illuminatingthe surface using an optical transmitter and recording the amount oflight reflected by, or transmitted through, the surface using an opticaldetector.

The step of analysing data received from the sensor unit using aprocessor to determine the integrity of the surface may compriseidentifying any disruption or attenuation in the light recorded by theoptical detectors.

Optionally, the method may include selecting the angle of incidence oflight output by an array of optical transmitters such that the lightundergoes total internal reflection between an outer and inner ply (orlayer) of glass.

In some embodiments, the method may include activating the opticalsystem in response to the microphone, camera (or other sensor in thesensor unit) recording a signal indicative of a potential damage event.Thus, the optical system may be used in the verification step.

In some embodiments, the method may include disposing an electricallyconductive film or coating on the surface. The method may includemeasuring the electrical resistance of the film and identifying anychanges to the electrical resistance of the film.

In some embodiments, the method may include activating the electricalcircuit which measured the resistance of the film in response to themicrophone, camera, optical system (or other sensor in the sensor unit)recording a signal indicative of a potential damage event. Thus, theconductive film may be used in the verification step.

In some embodiments, the method may comprise measuring the acousticproperties of the surface. For example, the method may include vibratingthe surface by outputting acoustic waves from a transducer. The methodmay also measuring the vibration of the surface using a receiver anddetecting any changes to the output acoustic signal, such as to thefrequency spectrum, which are indicative of damage to the surface.

Optionally, the method may include repeating the vibration analysis fora given number of times over a given amount of time, so as to confirmthat the changes to the acoustic signal are not temporary.

The method may include activating the acoustic vibration system inresponse to the microphone, camera, optical system, or conductive film(or other sensor in the sensor unit) recording a signal indicative of apotential damage event. Thus, the acoustic vibration system may be usedin the verification step.

Optionally, the method may comprise storing data output from one or moreof the sensor unit, the processor and/or the communication unit.

Optionally, the method may include outputting a signal (e.g. an alert)via the communication unit to a remote location. The remote location maybe a repair centre which can then automatically schedule the surface tobe repaired.

The communication unit may comprise a transceiver for outputting thesignal, for example an RF transceiver. Optionally, the communicationunit may be configured to output a signal via a wireless data link.

Optionally, the method may comprise determining the location of thenearest repair centre, or nearest designated repair centre, andoutputting the signal (or alert) to that location.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example,in the accompanying drawings, of which:

FIG. 1 is a schematic drawing of a detection system according to anembodiment of the invention;

FIG. 2 is a schematic drawing of another embodiment of a detectionsystem including an optical system;

FIG. 3 is a schematic drawing of another embodiment of a detectionsystem including a microphone and a camera; and

FIG. 4 is a schematic drawing of another embodiment of a detectionsystem including a camera and an acoustic vibration system.

DETAILED DESCRIPTION

FIG. 1 shows a schematic drawing of a damage detection system of thepresent invention. The system comprises a sensor unit 14 disposedproximate a glass surface 10. The sensor unit 14 is in communicationwith a processor 16. The processor 16 is connected to a communicationunit 18.

The sensor unit 14 is configured to measure one or more properties orone or more signals indicative of a damage event occurring to the glasssurface 10. A damage event includes a flaw or chip forming on the outerface of the surface 10. The processor 16 is configured to analyse datareceived from the sensor unit 14 and to determine when the surface 10has been damaged. When a damage event is detected the processor 16 caninstruct the communication unit 18 to output a signal, such as an alert,to a remote location 1. The communication unit 18 is in wirelesscommunication with the remote location (e.g. via a data link) shown bythe broken line in FIG. 1 .

Although the connections between the sensor unit 14, processor 16 andcommunication unit 18 are shown by solid lines in FIG. 1 , these mayalso be wireless connections.

In some embodiments, the processor 16 may request verification of adamage event from the sensor unit 14 before instructing thecommunication unit 18.

In FIG. 2 , the glass surface is a vehicle windscreen 20. The windscreen20 comprises an outer ply of glass 21 and an inner ply of glass 23separated by an interlayer 22 comprising a plastics material. Theinterlayer 22 is generally thinner than the inner 21 and outer 23 glassplies, but the layers are not drawn to scale in FIG. 2 to improve theclarity of the illustration.

The sensor unit in FIG. 2 comprises an array of optical transmitters 24and an array of optical detectors 25. A controller 27 is incommunication with the array of optical transmitters 24 and theprocessor 26. The processor 26 receives data from the opticaltransmitters 24 and the optical detectors 25. As in FIG. 1 , theprocessor 26 is connected to a communication unit 28 which is configuredto output a signal in response to the processor determining that thewindscreen 20 has been damaged.

The optical transmitters 24 are arranged to output dispersed light 29which undergoes total internal reflection through the interlayer 22 ofthe of the windscreen. The wavelength of the light emitted 29 may be inthe infrared, visible or ultraviolet spectrum. The angle of incidence αof the incident light 29 on the boundary of the interlayer 22 determineswhether total internal reflection will occur. The angle α can beadjusted by the controller 27 moving or tilting the optical transmitters24. In other embodiments, the optical transmitters 24 are fixed inposition and there is no controller 27.

The array of optical detectors 25 are positioned to receive light outputby the optical transmitters 24. In FIG. 2 , the optical detectors arearranged proximate an end of the surface 20 to receive light output fromthe interlayer 22. If the interface between the interlayer 22 and theouter ply 21 or the inner ply 23 of the windscreen becomes damaged, forexample by a chip, then this will disrupt the amount of light receivedby the detectors 25. This change will be detected by the processor 26which will then instruct the communication unit 28 to output an alert.

In other embodiments, the array of optical transmitters 24 may bearranged to at least partially illuminate the outer ply 21 of thewindscreen. The light emitted 29 may undergo total internal reflectionalong the outer ply 21. This may allow the optical detectors 25 todetect damage to the external face of the outer ply 21, which does notpropagate to the interlayer 22. Optionally, the optical transmitters 24and/or the optical detectors 25 may be positioned proximate the outerply 21 (i.e. externally of the windscreen 20). Optionally, the opticaltransmitters 24 and/or the optical detectors 25 may be positionedproximate to the inner ply 23 (i.e. inside the vehicle).

In FIG. 3 , the sensor unit of FIG. 2 has been replaced by a microphone34 positioned proximate to the windscreen 20 and a camera 35. Themicrophone 34 is arranged to convert sound signals into electronicsignals which are received by the processor 26. The processor 26includes a signal amplifier and a signal processor. The processor 26 isprogrammed to analyse whether the signal received from the microphone 34matches, or approximates a predetermined signal indicative of a damageevent. For example, the sound of a stone or other small projectileimpacting the outer ply 21 and causing a chip will result in a signaloutput from the microphone 34 which has a particular time response andfrequency spectrum.

There is a risk that the microphone 34 may pick up sounds from insidethe vehicle, or of objects striking the outer ply 21 of the windscreenwhich do not result in any damage but are identified by the processor 26as damage events. Thus, the camera 35 may be used to verify the resultsof the microphone 34.

When the processor 26 identifies a possible damage event from themicrophone 34 signal, it will activate the camera 35. The camera 35 maybe on standby until activated by the processor 26, so as to conservepower. If required, a controller 27 may move, tilt or pivot themicrophone 34 and/or the camera 35, such that the camera 35 can imagethe entire surface 23 of the windscreen. The image, or images, is/arethen received by the processor 26 which includes image processingsoftware configured to analyse the image(s) and identify any damageareas on the windscreen 20. As the outer ply 23, interlayer 22 and outerply 21 are transparent, the camera 35 can be positioned inside of thevehicle and still image any damage areas in the outer ply 21.

If the processor 26 determines that there is a damage area (e.g. a chip)then the processor 26 will instruct the communication unit 28 to outputan alert.

In FIG. 4 , an electrically conductive film 31 is disposed on theexternal face of the outer ply 21 (i.e. the external face of thewindscreen 20). The film 31 is connected to an electric circuit 44operable to measure the electrical resistance of the film 31. Inaddition to the electric circuit 44 the sensor unit comprises a camera35 (as in FIG. 3 ) and an acoustic vibration system 45.

One or more of the features of the sensor unit may be in continuousoperation. For example, the conductive film 31 and the acousticvibration system 45 may be used as the primary detection means.

The processor 26 is configured to compare the electrical resistance ofthe film 31 (measured by circuit 44) to a predetermined resistance ofthe film 31 when it is completely intact. The ‘undamaged’ resistancevalue may be measured by the circuit 44 when the film 31 is initiallyapplied to the windscreen 20. Any damage to the conductive film 31 (suchas a discontinuity) caused by damage to the outer ply 21 of thewindscreen will change the electrical resistance of the film 31. If theprocessor 26 determines that the resistance of the film 31 has changedthen it may verify that a damage event has occurred using the camera 35(as described above) and/or the acoustic vibration system 45.

The acoustic vibration system 45 comprises a transducer configured tooutput pulses of acoustic waves which vibrate the windscreen 20 and areceiver arranged to measure the vibration of the windscreen 20. In thisembodiment the transducer and the receiver are the same piezoelectricdevice 44, but in other embodiments they can be separate.

When undamaged the windscreen 20 will vibrate with a specific acousticsignal and temporal profile that can be predetermined. When thewindscreen 20 is damaged the signal received by the receiver 44 will bepermanently altered. The processor 26 analyses the signals recorded bythe receiver 44 and detects if the signal has changed from the undamagedsignal.

If the processor 26 receives multiple acoustic signals indicative of adamage event in a certain time frame (i.e. that the change was nottemporary) then the processor may analyse whether the electricalresistance of the film 31 has changed. If the film 31 verifies that adamage event has occurred then the processor 26 may instruct thecommunication unit 28 to output an alert.

Alternatively, the processor 26 may seek further verification of damagefrom the camera 35. The operation of the camera 35 is described above inreference to FIG. 3 .

This two-step verification system ensures that the detection of damageto the windscreen is very accurate and reliable compared to knownsystems.

It will be appreciated that the sensor unit could comprise anycombination of the features disclosed in FIGS. 2 to 4 , and that systemsdisclosed as providing verification could equally be used as primarydetection means, and vice versa. As such, the sensor unit of the presentinvention may comprise any one or more of: a microphone 34; a camera 35;an optical system 24, 25; a conductive film 31 and electric circuit 44;and/or an acoustic vibration system 45. For example, the optical systemof FIG. 2 could be used in the sensor units of FIG. 3 or FIG. 4 , eitherto provide the initial indication of a damage event, or to verify thediagnosis of one of the other components of the sensor unit.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe capable of designing many alternative embodiments without departingfrom the scope of the invention as defined by the appended claims. Inthe claims, any reference signs placed in parentheses shall not beconstrued as limiting the claims. The word “comprising” and “comprises”,and the like, does not exclude the presence of elements or steps otherthan those listed in any claim or the specification as a whole. In thepresent specification, “comprises” means “includes or consists of” and“comprising” means “including or consisting of”. The singular referenceof an element does not exclude the plural reference of such elements andvice-versa. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage.

The invention claimed is:
 1. A system for detecting damage to a glasssurface, the system comprising: a sensor unit disposed proximate thesurface; a processor in communication with the sensor unit, wherein theprocessor is configured to analyze data received from the sensor unit todetermine the integrity of the surface; and a communication unitconfigured to output a signal in response to the processor determiningthat the surface has been damaged; wherein the sensor unit comprises acamera arranged to image the surface and the processor comprises imageprocessing software operable to analyze images received from the camerato identify any damage areas; and wherein a controller is configured tomove, tilt or pivot the camera.
 2. A damage detection system accordingto claim 1, wherein the sensor unit comprises a microphone and theprocessor comprises a sound amplification and signal processing system.3. A damage detection system according to claim 1, wherein the sensorunit comprises an optical transmitter and an optical detector, andwherein the optical transmitter is configured to at least partiallyilluminate the surface and the optical detector is arranged to at leastpartially receive light output from the optical transmitter.
 4. A damagedetection system according to claim 3, wherein the angle of incidence oflight output by the optical transmitter(s) is selected such that thelight undergoes total internal reflection through the surface.
 5. Adamage detection system according to claim 1, further comprising anelectrically conductive film disposed on the surface.
 6. A damagedetection system according to claim 5, wherein the sensor unit comprisesan electric circuit operable to measure the electrical resistance of thefilm.
 7. A damage detection system according to claim 1, wherein thesensor unit comprises a transducer configured to output acoustic waveswhich vibrate the surface and a receiver arranged to measure thevibration of the surface.
 8. A damage detection system according toclaim 7, wherein the transducer and the receiver are the same devicewhich is configured to output pulses of acoustic waves.
 9. A damagedetection system according to claim 1, wherein the communication unit isconfigured to output an alert that the surface has been damaged to aremote location.
 10. A damage detection system according to claim 9,wherein the communication unit comprises at least one of a transceiverfor outputting the signal, a GPS transmitter, and a GPS receiver.
 11. Amethod for detecting damage to a glass surface, the method comprising:measuring one or more acoustic, electrical, or optical properties of, orproximate to, the surface using a sensor unit; analyzing data receivedfrom the sensor unit using a processor to determine the integrity of thesurface; and outputting a signal from a communication unit in responseto the processor determining that the surface has been damaged; whereinthe sensor unit comprises a plurality of sensors, and the methodincludes the step of instructing the sensor unit to verify whether thesurface has been damaged by conducting a further measurement using adifferent sensor.
 12. A damage detection method according to claim 11,wherein the glass surface is a vehicle windscreen.
 13. A damagedetection method according to claim 11, wherein the step of measuringone or more properties using the sensor unit comprises recording thesound of a potential damage event using a microphone and converting thissound signal into an electrical signal.
 14. A damage detection methodaccording to claim 11, wherein the step of measuring one or moreproperties using the sensor unit comprises imaging the surface using acamera; wherein the method includes moving the camera to scan across thesurface.
 15. A damage detection method according to claim 11, whereinthe method includes verifying that a damage event has occurred byactivating a camera in response to a microphone recording a signalindicative of a potential damage event.
 16. A damage detection methodaccording to claim 11, wherein the step of measuring one or moreproperties of the surface using a sensor unit comprises at leastpartially illuminating the surface using at least one opticaltransmitter and recording the amount of light reflected by, ortransmitted through, the surface using at least one optical detector.17. A damage detection method according to claim 11, further comprisingselecting the angle of incidence of light output by the opticaltransmitter(s) such that the light undergoes total internal reflectionthrough the surface.
 18. A damage detection method according to claim11, including disposing an electrically conductive film on the surfaceand measuring the electrical resistance of the film.